The present invention relates to a voltage standard employing an array of dc SQUID's (Superconducting Quantum Interference Devices) and to a D/A converter, using the arrays of dc SQUID's.
There are several ways to build Josephson effect voltage standards. The conventional Josephson effect standard relies on millimeter-wave injection locking of a series array of Josephson junctions. (See C. A. Hamilton, C. J. Burroughs, and Kao Chieh, Operation of Josephson array voltage standards, NIST J.Res., vol.95, pp. 219-235, April 1990.) In the conventional scheme the main problem is the stabilization of the single junction against chaotic behavior which results in frequent switching between different Shapiro steps. To solve this problem the resistive leakage must be very small and the capacitance to critical current ratio C/I.sub.c must be very large, so that f/f.sub.p &gt;&gt;1, where the millimeter-wave frequency f is typically 75-90 GHz, the junction plasma frequency f.sub.p =(eI.sub.c /.pi.hC).sup.1/2, e is the electric charge, I.sub.c is the junction critical current, h is Planck's constant, and C is the junction capacitance. The junctions also must be rather large to achieve thermal stability, but not so large that resonant effects occur. These three factors place severe constraints even on the most reliable high quality niobium/aluminum oxide/niobium technology. Besides these limitations, frequency sources in the 75-90 Ghz range are very expensive and have a very high noise to signal ratio.
Therefore, an alternative to the conventional Josephson effect standard is desired.
Semenov et al. in DC Voltage Multipliers: a novel application of synchronization in Josephson junction arrays, IEEE Trans. Magn., vol.25, pp. 1432-1435, March 1989, and Hamilton in Josephson Voltage Standard Based on Single-Flux-Quantum Voltage Multiplies, unpublished NIST proposal, have independently proposed a "digital" approach to a Josephson effect voltage standard. The Semenov-Hamilton scheme offers the attractive possibility of using a very low frequency external source. According to this approach a sinewave at a relatively low frequency f.sub.s =500 Mhz is converted into a train of positive single-flux-quantum (SFQ) pulses by the single-junction SQUID. The amplification of the pulse train through additional transformers is accomplished by a flux shuttle (or Josephson transmission line-JTL). This low frequency in the Hamilton proposal is multiplied 128 times by using an array of 128 junctions to dc voltage bias a single junction master oscillator operating at 128f.sub.s. This high frequency on-chip oscillator drives JTL's which lock larger arrays to generate 10 Volts.
This approach, however, is also impractical. First, there are a large number of junctions in the JTL which are in dc parallel so that an enormous amount of dc current is required to drive the JTL. This big current results in a high magnetic field, which can destroy the superconductive state of the Josephson junction. Secondly, the JTL's transmit a pulse-train waveform that has a high amount of harmonic content so that the fundamental I-V Shapiro step on which the array junctions are supposed to lock is severely diminished. This can lead to the junction's chaotic transitions between different Shapiro steps. In general, if a single Josephson junction is effectively biased by an rf (radio-frequency) current source, the Shapiro steps are an order of magnitude less than the "text-book" Shapiro steps, because the text-book Shapiro steps require an rf voltage source for the biasing of the single Josephson junction. The text-book situation for the Shapiro steps is never achieved either in the conventional or in the Semenov-Hamilton "new" approach for the Josephson effect voltage standard.
Thus, it is desirable to provide a practical and improved Josephson junction voltage standard.